A wiring diagram is a simple visual representation of the physical connections and physical layout of an electrical system or circuit. It shows how the electrical wires are interconnected and can also show where fixtures and components may be connected to the system.. PSI specializes in the design and manufacture of Standalone Wiring Harnesses for GM Gen II, III, and IV LS based engines and transmissions. These harnesses include the Gen II LT1/LT4, Gen III (24x) LS1/LS6 and Vortec Truck Engines as well as Gen IV (58x) LS2, LS3 and LS7 Engines.. Mar 18, 2011 · I don't think the coil wiring would affect anything much, as the USDM harness already has the provisions for the external coil. This is what I wanted to know. I wasn't asking if i could plug rywire's harness into my usdm obd2 engine harness and then plug the other end of rywire's harness into the jdm obd2 dizzy and be all set to go..